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High-strength mix on a spiral of success

BUILDING STUDY

Danish scientists have developed an ultra-high-strength concrete that allows the material to be used with a slenderness and elegance previously considered unachievable

Company House on Tuborg Boulevard in Hellerup, a suburb of Copenhagen, is a shrine to the wonders of a new concrete. This four-storey rectangular office block with its cool white fascia panels and glass-lined external elevations gives no hint of the surprise awaiting any visitor making their way to the central atrium to grab a coffee in the restaurant or check in at reception. Standing four floors tall, looking like a paper streamer hanging from the ceiling, is a pencil-thin, smooth, white, spiral staircase. It seems impossible that anything so slender, so elegant and so dramatically curved could possibly be cast in concrete. Surely it must be steel or aluminium? You take a closer look, you walk up the elegant fan-tailed flights, but you can still scarcely believe that this really is concrete.

In the past 10 years, research organisation CRC Technology in Aalborg, Denmark, has developed an ultra-high-strength cement mortar that can be mixed and poured in place just like conventional concrete. It is called Compact Reinforced Composite (CRC). Strengths of up to 300Mpa for precast applications are achieved by controlling the particle-size distribution of the cement to minimise the void spaces between the cement grains, reducing macro defects in its crystalline structure, and giving the material its ultra-high compressive strength. Very finely divided silica sand is added to the cement, filling the voids left by the cement particles. There is no coarse aggregate. The silica has the added advantage of reacting chemically with the cement paste to become an integral part of the matrix. A dispersion surfactant or superplasticiser is also necessary to achieve workability for placing, as this is an earth-dry mix with a water/cement ratio as low as 0.16. Steel fibres are added to give this brittle material toughness and ductility. The quantity of steel fibre will vary from 2-6 per cent by volume of concrete, depending on the strength required and the material's application. The spiral staircase in Company House, which is owned by Tuborg Nord, has a modest characteristic compressive strength of 100Mpa and a design tensile strength of 5Mpa - only a 'cooking' grade for CRC.

The staircase, which provides access to all floors, is along the west elevation of the building atrium and strategically positioned so that it can be accessed easily by employees of all three companies that occupy the building. It has been conceived as a sculpture and focal point of the atrium area, which contains the main foyer and staff restaurant. The spiral of the staircase has an outer diameter of 6m.

Steps cantilever from an inner support wall that also acts as a banister. The inner wall and tapering cantilever steps are precast monolithically in four segments, to form a complete spiral between floors. The thickness of the steps at the outer edges has been reduced to 30mm - from the 100mm depth at the root - for aesthetic reasons. The outer banister is a wall of clear glass, while fibre-optic lighting moulded to the inner CRC wall provides soft accent lighting.

The main structural element of the staircase is the inner wall which acts as a spiral beam, 1,500mm high and 150mm thick, spanning between floors.

The spiral beam has a diameter of 1.5m and makes one revolution per floor. The steps cantilever 2.25m from the spiral beam, with live loads carried by the steps transferred to the central column support at landing level via bending and torsion in the spiral beam. The staircase has been precast in 16 repeating elements, each with six steps. There are four landing sections and four central column pieces. Four staircase elements, one landing and one central column element are required to complete each rise of floor.

A conventional pan mixer can be used for mixing CRC.

The mixing time is between five and eight minutes after the water is added. The bundles of steel fibres are added part-way through the mixing. The material is sensitive to temperature change, as the high dosage of superplasticiser tends to retard the hardening, and should not be used below 5infinityC. At 20infinityC the initial set will start after seven to eight hours, with a compressive strength of 60Mpa achieved after 16 hours, for a typical 200Mpa design mix. Because of the very low water content, the surface must be covered as quickly as possible in hot weather and drying winds to prevent evaporation, and the precast element enclosed by tenting with tarpaulins. Surface finishing is a problem as the mix is very sticky.

A spike roller, normally used for screeding, is quite effective for levelling the surface.

As the steel fibres are incorporated only to impart ductility to the CRC, normal bar reinforcement is necessary to resist the tensile and torsional forces generated by the applied loads, to control deflection and any drying shrinkage movement.

The design E-value for the composite section was taken conservatively as 30Gpa, although it is has been taken as 50Gpa for the higher grades of CRC. The staircase was designed in accordance with Danish codes of practice and is fire resistant for up to 30 minutes.

The precast elements were cast at the Beton-Tegl works, transported to the site and placed on a scaffold support before being jointed together using a special mix of CRC that acts like a cement weld. If the microsilica and cement content are raised and the dosage of steel fibre increased to 6 per cent, CRC becomes a powerful bonding glue where rebar only needs a bond length of eight diameters for a full tension lap. To satisfy the need to be able to pour and compact the cement mortar, the joint spacing for the spiral staircase was kept to 100mm. For aesthetic reasons and to achieve a uniform appearance, the midgrey colour of the CRC and the ugly stitches of the joints were disguised with a coat of white concrete paint, and the cantilevered steps covered in hardwood.

In terms of cost, the high concentration of cement and the inclusion of steel fibres makes CRC expensive - as much as £500/m 3.But the reduction in section depth of a CRC structure like the Tuborg staircase means a much smaller volume of concrete is required, typically only about a third of the volume of conventional concrete. For balcony slabs and staircase applications in Denmark, the price of CRC structures is equivalent to that of steel.Where CRC has been engineered to its full potential and, for example, a balcony slab made to cantilever the span without the need for a supporting column, contractors have found it to be the cheapest option.

While compressive strength and ductility are greatly enhanced in CRC, stiffness is only slightly higher than for normal concrete, so deflection has to be considered carefully and controlled by reinforcement. Allowances also have to be made for drying shrinkage, due to the high cement content, and that is catered for by detailing additional reinforcement.

The incredible properties and challenge of CRC have certainly fired the imagination of architects and engineers in Denmark, leading to an increasing number of innovative and pioneering structures like this spiral staircase.

It should not be long before we see a similar trend in the UK.

There is already a lot of interest, but it remains to be seen which team of architects and engineers will be the first to employ the material.

CRC: HIGH-PERFORMANCE CONCRETE FOR PRECAST APPLICATIONS

Bendt Aarup, CRC Technology High-strength or high-performance concretes (HSC or HPC) are being used increasingly for a range of structural applications, and standards in a number of countries are being revised to accommodate these improved materials. Often, however, HSC is more brittle than conventional concretes, which can lead to problems in failure mode as well as under service conditions. One way of overcoming this problem is to provide ductility by incorporating steel fibres in the matrix.

CRC - a special type of fibre-reinforced, high-performance concrete developed in 1986 by Aalborg Portland in Denmark - incorporates fibre contents of 2-6 per cent by volume, corresponding to between 150kg and 475kg of steel fibres per cubic metre of concrete. In addition to this, the matrix has a very large content of micro silica and water/binder ratios of typically 0.16 or lower. This composition makes CRC very dense and well suited to structural applications, with a typical mean compressive strength of 150N/mm 2, high tensile strength, good durability and a high ductility. It is thus possible to use reinforcement much more effectively without having large cracks under service conditions.

As CRC is quite different from conventional concrete, extensive documentation on its properties needed to be provided before the material could be considered for structural applications. CRC has been the subject of a number of research projects dealing with structural properties but also with aspects such as durability and fire resistance.

With the high fibre content somewhat detrimental to workability, CRC is particularly suitable for precast applications. A special type of CRC called CRC JointCast - a mortar with 6 per cent of fibres (475kg/m 3) - has been used for in situ cast joints between structural members of ordinary concrete. The Building Research Establishment at Watford investigated this application in a project on innovative joints under a DETR Framework Programme. For precast applications, a special CRC binder is sold and the production plants are then given advice on casting and which aggregates to use, etc. CRC JointCast is supplied as a dry mortar, mainly to improve quality control because it is typically used on site.

Due to the low water content, CRC has to be protected from evaporation shortly after casting, and the high content of super plasticiser means that at normal temperatures (20degreesC) it takes up to 16 hours before elements can be removed from the moulds. Typical strength at one day is 80N/mm 2.Usual applications for CRC in precast production are small, slender elements such as balcony slabs, staircases, beams and columns. While strength and ductility are much better than for conventional concrete, stiffness is only slightly higher, which means that deformation is one of the aspects that has to be considered carefully in design. Allowance also has to be made for a large shrinkage due to the large binder content. This is usually handled by incorporating additional reinforcement.

CREDITS

CLIENT Tuborg Nord

ARCHITECT Arkitema K/S

STRUCTURAL ENGINEER Ramboll

MAIN CONTRACTOR NCC Denmark

PRECAST STAIRCASE Beton-Tegl

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